As a method for obtaining a magneto-optical recording medium with higher recording density, a double-mask rear aperture detection (D-RAD) employing the magnetic super resolution (MSR) technology is known. In the D-RAD scheme, a front mask and a rear mask are formed before and behind a mark recorded on the magneto-optical recording medium, thereby virtually narrowing a spot diameter of a laser beam radiated on the magneto-optical recording medium to improve recording and reproduction resolution.
FIG. 10 shows a diagram illustrating the reproduction principle in the D-RAD scheme. In FIG. 10, the magneto-optical recording medium corresponding to the D-RAD has a three-layer structure consisting of recording layer, intermediate layer and reproduction layer. Rotating such a magneto-optical recording medium produces temperature distribution in a laser spot S, forming a low-temperature zone, a high-temperature zone, and a medium-temperature zone existent therebetween. When reproducing from the medium, applying a bias magnetic field (reproduction magnetic field) Hr produces dominant magnetization in the intermediate layer in the low-temperature zone, causing magnetization in the intermediate layer aligned in the direction of the bias magnetic field Hr. At the same time, magnetization in the reproduction layer is also aligned so as not to form an interface magnetic wall, and thus a front mask is generated. In contrast, in the high-temperature zone, magnetization in the reproduction layer becomes dominant. This produces magnetization in the reproduction layer aligned with the bias magnetic field Hr, and thus a rear mask is generated. There is an aperture in the medium-temperature zone existent between the front mask and the rear mask, and a bit in the recording layer is transcripted to the reproduction layer through the intermediate layer, and the bit is read out. In such a way, because the bit is reproduced only from the medium-temperature zone (aperture) in the laser beam spot S, the spot diameter becomes virtually narrower, and thus improved reproduction resolution is obtained.
FIG. 11 shows a diagram illustrating positional relation between a bias magnetic field generator and a laser beam spot in the conventional magneto-optical recording medium device. In FIG. 11, a laser beam is irradiated from one face side of the magneto-optical recording medium, while the bias magnetic field is applied from the other face side by the bias magnetic field generator. The bias magnetic field generator is constituted of an electromagnet having a coil wound around a yoke. Further, a cross section of the yoke illustrated (the cross section on the radius direction) is structured left-right symmetric against the center line C1 of the yoke, as an example, with a rectangular form. Further, the bias magnetic field generator is disposed so that the center of the irradiated laser beam spot is aligned on the same line as the center line C1.
FIG. 12 is a diagram illustrating relation between the distance from the center of the yoke and the magnitude (ratio) of the generated bias magnetic field. In FIG. 12, the peak location (100%) of the bias magnetic field is aligned on the same line as the center line of the laser beam spot. Accordingly, the bias magnetic field becomes left-right symmetric before and behind the track in the medium rotation direction positioned at the center of the laser beam spot. This produces identical magnitude of the magnetic field forming the front mask to the magnitude of the magnetic field forming the rear mask.
In such a way, in order to detect microscopic bits recorded in high density, the front mask and the rear mask are produced by the bias magnetic field, as shown in FIG. 10. Here, in the front mask, it is necessary to align the reproduction layer orderly in the ERASE direction. For this purpose, it is desirable that the magnitude of the magnetic field forming the front mask be comparatively large. Further, as for the size of the rear mask, it is necessary to limit the size within a predetermined value from the viewpoint of crosstalk with the neighboring track. Therefore, it is desirable that the magnitude of the magnetic field forming the rear mask be comparatively small. If the magnetic field forming the rear mask is set too large in magnitude, an aperture is produced also in the neighboring track because of the extended rear mask area, causing crosstalk by which the bit on the neighboring track is reproduced.